1. Field of the Invention
The present invention relates to a planar inductor applied to, e.g., a DC-to-DC converter.
2. Description of the Related Art
A conventional ferrite troidal coil has been used as a choke coil on the output side of, e.g., a DC-to-DC converter. In contrast to this, a planar inductor has been recently studied in order to achieve miniaturization of an apparatus.
For example, a planar inductor with a structure having a spiral or meander planar coil, insulating layers stacked on both surfaces of the planar coil, and ferromagnetic layers stacked on the insulating layers is known.
In order to obtain high inductance, an amorphous alloy ribbon having a high permeability is used as a ferromagnetic layer. Note that many amorphous alloys have a positive saturation magnetostriction. Thus, when an amorphous alloy having a saturation magnetostriction is used as a normal troidal magnetic core, complicated magnetic anisotropy occurs during a heat treatment for eliminating strain by an inverse magnetostrictive effect due to a flexural stress, and soft magnetic properties such as an effective permeability are degraded. On the other hand, when an amorphous alloy is applied to a planar inductor, a ribbon of the alloy is used in a planar state. Therefore, the above-mentioned degradation of soft magnetic property due to an inverse magnetostrictive effect is small, and the soft magnetic property of the alloy can be sufficiently utilized. Therefore, in the troidal magnetic core and the planar inductor, a ferromagnetic ribbon need not be treated in the same manner.
When the planar inductor is applied to a choke coil on the output side of, e.g., a DC-to-DC converter, a high-frequency current superposed with DC current is supplied to the planar inductor. Therefore, excellent DC superposition characteristics are required.
The conventional planar inductor, however, undesirably has poor DC superposition characteristics. This problem is caused because the magnetic characteristics of a ferromagnetic ribbon which has been conventionally used are inadequate. More specifically, in the planar inductor, a magnetic flux flows in a plane of a surface of the ferromagnetic ribbon. When the saturation magnetization of the ferromagnetic ribbon is low, however, even if a small DC magnetic field is superposed, a magnetic flux density is saturated. Although the ferromagnetic ribbon having a high permeability is used in order to obtain higher inductance, an inductance is reduced, thus degrading DC superposition characteristics. For example, a ferromagnetic ribbon having a high permeability consisting of a Co-based amorphous alloy is known, and its saturation magnetization is higher than that of a ferrite. However, this saturation magnetization is insufficient to prevent a reduction in inductance, and the DC superposition characteristics are degraded.
Assume that a Co-based amorphous alloy is used as a ferromagnetic ribbon. If the Co-based amorphous alloy ribbons are stacked, the DC superposition characteristics can be improved to some extent. However, if a large number of amorphous alloy ribbons are stacked, the thickness of the planar inductor is increased. Therefore, in consideration of an object to obtain a thin planar inductor, stacking a large number of amorphous alloy ribbons is not preferable.
If the DC superposition characteristics of the planar inductor are poor, a inductance is reduced, and a control becomes difficult. Accordingly, the efficiency of a DC-to-DC converter is lowered. Thus, it is inadequate to apply the planar inductor directly to, the DC-to-DC converter and the like. Therefore, in order to improve the DC superposition characteristics, a high saturation magnetization of a ferromagnetic ribbon having a high permeability is required.
Even if the DC superposition characteristics on the inductance can be improved, an improvement of the efficiency of the DC-to-DC converter to which the planar inductor is applied is limited due to a high-frequency loss of the ferromagnetic ribbon. Therefore, in order to obtain a high efficiency equivalent to that of a conventional ferrite troidal coil, a high-frequency loss of the ferromagnetic ribbon must be decreased.
In addition, the planar inductor is used in practice while being coated with a mold resin. For this reason, if the amorphous alloy ribbon has a positive saturation magnetostriction, when the surface of the planar inductor is coated with a liquid mold resin and the resin is hardened, a compressive stress is applied to the ferromagnetic ribbon upon contraction of the mold resin. An effective permeability is then decreased due to an inverse magnetostrictive effect, thus reducing an inductance.